The following may be listed as specific examples of exhaust gas catalysts for internal combustion engines and automotive engines in particular: oxidation catalysts for simultaneously treating hydrocarbon (HC) and carbon monoxide (CO) in exhaust gas; three-way catalysts for simultaneously treating the hydrocarbon, carbon monoxide and nitrogen oxides (NOx) in exhaust gas; NOx occlusion-reduction catalysts, wherein NOx is occluded from the exhaust gas at lean air-fuel ratio, the air-fuel ratio is switched to the theoretical air-fuel ratio or a rich air-fuel ratio before the occluded NOx reaches saturation, and the NOx is thereby reduced; and NOx selective reduction catalysts that reduce the NOx in the exhaust gas by means of a reducing agent.
Oxidation catalysts, three-way catalysts, NOx occlusion-reduction catalysts and NOx selective reduction catalysts are manufactured by making the catalyst components into a slurry, soaking onto a support such as a ceramic support of honeycomb geometry and calcining.
In nearly all existing exhaust gas catalysts, however, the catalyst components are formed uniformly on the support; gas diffusion in the exhaust gas is therefore slow when the exhaust gas flows into the exhaust gas catalyst, and as a result the exhaust gas is often found to be incompletely treated.
In view of this, Japan Patent Kokai No. 2002-191988 (Patent Reference 1) and Japan Patent Kokai No. 2002-253968 (Patent Reference 2) have proposed an NOx occlusion-reduction catalyst wherein the gas diffusibility of the exhaust gas is raised and the NOx cleaning efficiency is enhanced by supporting a noble metal and NOx occluding agent in a coating layer comprising a porous structure provided with pores of a specified size. However, there remains a need to raise the gas diffusibility of exhaust gas in the layer of catalyst components in exhaust gas catalysts to improve the efficiency of treatment of the exhaust gas.    Patent reference 1: Japan Patent Kokai No. 2002-191988    Patent Reference 2: Japan Patent Kokai No. 2002-253968